Chapter 10

1. Chapter 10 Sensory Physiology 10-1

2. Sensory Receptors • Transduce (=change) environmental info into APs -- the common language of NS • Each type responds to a particular modality (=form of info, e.g. sound, light, pressure) • Different modalities perceived as different because of CNS pathways they stimulate 10-4

3. Sensory Receptors continued • Can be simple dendritic endings of neurons • Or specialized endings of neurons or non-neuronal cells 10-5

4. Sensory Receptors continued • Are grouped according to type of stimulus they transduce • Chemoreceptors sense chemical stimuli • Photoreceptors transduce light • Thermoreceptors respond to temperature changes • Mechanoreceptors respond to deformation of their cell membrane • Nociceptors respond to intense stimuli by signaling pain • Proprioceptors signal positional info of body parts 10-6

5. Law of Specific Nerve Energies • Stimulation of sensory fiber evokes only the sensation of its modality • Adequate stimulus is normal stimulus • Requires least energy to activate its receptor 10-9

6. Cutaneous Sensations 10-13

7. Cutaneous Sensations • Include touch, pressure, heat, cold, pain • Mediated by free and encapsulated nerve endings • Free nerve endings mediate heat, cold, pain 10-14

8. Cutaneous Sensations continued • Cold receptors located in upper dermis • Warm located deeper in dermis 10-15

9. Cutaneous Sensations continued • Pain mediated by nociceptors • Use glutamate and Substance P as NTs • Substance P called "pain NT" • Heat elicits pain thru capsaicinreceptors • Capsaicin is "hot" chemical in chili peppers 10-16

10. 10-19

11. Receptive Field • Is area of skin whose stimulation results in changes in firing rate of sensory neuron • Area varies inversely with density of receptors • E.g. back, legs have low density of sensory receptors • Receptive fields are large • Fingertips have hi density of receptors • Receptive fields are small 10-22

12. Two-Point Touch Threshold • Is minimum distance at which 2 points of touch can be perceived as separate • Measure of tactile acuity or distance between receptive fields 10-23

13. 10-25

14. Chemical Senses 10-26

15. Taste/Gustation • Detects sweet, sour, salty, bitter and aminoacids (umami) • Taste receptor cells are modified epithelial cells • 50-100 are in each taste bud • Each bud can respond to all categories of tastants 10-28

16. Vision 10-62

17. Vision • Eyes transduce energy in small part of electromagnetic spectrum into APs • Only wavelengths of 400 – 700 nm constitute visible light 10-63

18. Structure of Eye • Photoreceptors are in retina • Retina absorbs some light • Rest is absorbed by the dark choroid layer • Axons of retinal neurons gather at the optic disc (blind spot) and exit eye in optic nerve 10-66

19. Retina • Is a multilayered epithelium consisting of neurons, pigmented epithelium, and photoreceptors (rods and cones) • Neural layers are an extension of brain • Light must pass through several neural layers before striking rods and cones 10-75

20. Retina continued • Rods and cones face away from pupil • send sensory info to bipolar cells • Bipolars send electrical activity to ganglion cells • Ganglion cells project axons thru optic nerve to brain • Horizontal cells and amacrine cells are interneurons involved in visual processing in retina 10-76

21. Effect of Light on Rods • Rods are activated when light produces chemical change in rhodopsin • Causing it to dissociate into retinal and opsin • = bleachingreaction • Causes changes in permeability, resulting in APs in ganglion cells 10-79

22. Dark Adaptation • Is a gradual increase in photoreceptor sensitivity when entering a dark room • Maximal sensitivity reached in 20 min • Increased amounts of visual pigments produced in the dark • Increased pigment in cones produces slight dark adaptation in 1st 5 min • Increased rhodopsin in rods allows light sensitivity to increase up to 100,000-fold 10-80

23. Electrical Activity of Retinal Cells • Ganglion and amacrine cells produce APs; rods, cones, bipolar and horizontal cells produce graded potential changes • Visual transduction is inverse of other sensory systems • In dark, photoreceptors release inhibitory NT that hyperpolarizes bipolars • Light inhibits photoreceptors from releasing inhibitory NT, thus stimulating bipolars 10-81

24. Electrical Activity of Retinal Cells • Rods and cones contain many Na+ channels that are open in dark • This depolarizing Na+ influx is the dark current • Light hyperpolarizes by closing Na+ channels 10-82

25. Cones and Color Vision • Cones less sensitive than rods to light • Provide color vision and greater visual acuity • In day, high light intensity bleaches out rods, and high acuity color vision is provided by cones 10-85

26. Cones and Color Vision continued • Humans have trichromatic color vision • All colors created by stimulation of 3 types of cones • Blue, green, red • According to region of visual spectrum they absorb 10-86

27. Cones and Color Vision continued • Instead of opsin, cones have photopsins • A different photopsin for each type of cone • Causing each to absorb at different wavelengths 10-87